1. Field of the Invention
This invention is related in general to the field of optical filters. In particular, it relates to a tunable filter with an extremely narrow pass-band and a blocking stop-band extending beyond the visible spectrum of light.
2. Description of the Related Art
Various optical applications (such as imaging, detection of spectral signatures of chemical species, and remote sensing) are carried out at a very precise wavelength within the broad spectrum of sun light, which ranges from deep ultraviolet (UV) to far infrared (IR) radiation. For proper functioning, these applications require extremely narrow-band filtering of the incoming optical signal. The filtering efficiency determines the signal-to-noise ratio of the output and, therefore, the ability of the optical instrumentation to successfully perform its functions. For example, in lidars (radar-like optical instruments utilized for atmospheric measurements), the ability to select the required fixed narrow spectral bandwidth and also simultaneously block all background radiation (practically across the entire sunlight spectrum because the sun provides a very strong wide-band background) is of critical importance in view of the extremely low-level signals received by the instrument (in the order of photon counting, in some cases). The operational requirements are further complicated by the fact that the particular spectral line of interest may be de-tuned from the expected spectral position as a result of operational conditions (such Doppler shifts, for example). Currently, a single filter capable of meeting such demanding spectral characteristics does not exist.
Etalon devices are well known for producing a periodic comb-like spectral response with a period and peak wavelengths determined by the physical characteristics of the etalon. Thus, in the prior art, narrow-band filters for solar observation have been implemented using an etalon device combined with conventional thin-film filters designed to attenuate all signals except the spectral line of interest. As illustrated schematically in FIG. 1, an etalon consists of an optical cavity 10 with two reflective surfaces 12 and 14 having reflectivity R1 and R2, respectively. The desired reflectivity of the surfaces 12, 14 may be obtained in various equivalent ways, such as by coating either side of each surface. When a beam of light L impinges on the cavity 10, a portion of light RF is reflected out of the cavity while another portion TR is transmitted through the cavity. Because of multi-reflection interference in the cavity, both the reflected and the transmitted outputs have a periodic frequency spectrum and a shape that depends on the so-called “Finesse” of the etalon, a quantity that indicates the spectral selectivity of the etalon and can be calculated as a function of surface reflectivities. For example, for the case when R1=R2=R, the relationship between the reflectivity of the cavity and the width of the spectrum of the periodic wave produced on transmission by the cavity may be quantified by the following general equation:πR/(1-R)=FSR/FWHM=Finessewhere R=R1=R2 is the reflectivity of each reflective surface in the cavity, FSR is the cavity's free-spectrum range, and FWHM is the full width of the transmission normalized-frequency spectrum curve at half maximum.
The “free-spectrum range” of an etalon cavity is the ration c/(21), where c is the speed of light and 1 is the optical length of a cavity. Free-spectrum range also refers to the distance (measured in the normalized-frequency domain) between peaks in the comb-like spectrum of the output of the cavity. Moreover, the exact frequency position of each periodic peak also depends of the cavity's optical length. Thus, the period and the peak frequencies of the frequency spectrum obtained from the cavity can be adjusted by varying the optical length of the cavity.
These properties of etalons have been used advantageously in the past to produce very narrow-band filters for solar observation by combining the etalon cavity with thin-film blocking filters having a pass-band overlapping the wavelength of interest in the spectrum produced by the etalon. The problem with these composite devices is that the blocking efficiency of thin-film filters is reduced at wavelengths removed from the band of interest. As a result, the periodic spectral lines produced by the etalon are blocked with diminishing effectiveness at frequencies in the visible range away from the spectral line of interest. Moreover, no blocking at all is provided in the IR and UV ranges of wavelengths.
The human eye is very susceptible to damage from exposure to IR and UV wavelengths because they produce extremely harmful thermal and chemical effects on the retina, respectively. Therefore, when human observation of an incoming image is desired in an instrument such as a telescope, it is necessary to attenuate these wavelengths below acceptable levels. In the case of white light, attenuation by a factor of at least 10−5 is considered safe. Thus, the use of appropriate filters for very narrow-band observation is not only important for isolating the signal of interest from noise and unwanted background signals but is also extremely important for safety while observing very bright objects such as the sun.
Therefore, any tunable filter capable of passing a signal with narrow bandwidth approaching that of a spectral line of interest while blocking all other wavelengths across the visible as well as the UV and IR spectra would represent a very desirable advance in the art. This invention achieves these goals using a tunable etalon cavity combined with a variety of conditioning filters adapted to selectively block all radiation other than the single order of interest in the etalon spectrum.